水交换越来越被认为是一种重要的生物过程,可以影响使用扩散MR对生物组织的研究。衡量交换的方法,然而,与那些用来描述限制的人相比,仍然不成熟,在最佳脉冲序列或信号模型上没有共识。总的来说,趋势是高度参数化模型的数据密集型拟合。我们采取相反的方法,并表明,一个明智的子样本的扩散交换光谱(DEXSY)数据可以用来稳健地量化交换,以及限制,以数据高效的方式。该采样产生每个混合时间两个点的比率:(i)在两个编码周期中具有相等扩散加权的一个点,这给出了最大的交换对比,和(ii)仅在第一个编码周期中具有相同总扩散加权的一个点,用于正常化。我们称这个商为扩散交换比(DEXR)。此外,我们表明,它可以用来探测时间相关的扩散,通过估计速度自相关函数(VACF)在中间到长时间(~2-500ms)。我们为静态或恒定梯度情况下的DEXR实验设计提供了一个全面的理论框架。提供了使用永磁系统在固定和可行的离体新生小鼠脊髓中获得的蒙特卡罗模拟和实验数据,以测试和验证这种方法。在可行的脊髓中,我们仅从6个数据点报告以下表观参数:τk=17±4ms,fNG=0.72±0.01,Reff=1.05±0.01μm,和κeff=0.19±0.04μm/ms,对应于交换时间,受限或非高斯信号分数,有效球面半径,和渗透性,分别。对于VACF,我们报道了很长时间,具有≈t-2.4的幂律缩放,这与3-D中的无序域近似一致。总的来说,DEXR方法被证明是高效的,能够使用最少的MR数据提供有价值的定量扩散指标。
Water
exchange is increasingly recognized as an important biological process that can affect the study of biological tissue using diffusion MR. Methods to measure
exchange, however, remain immature as opposed to those used to characterize restriction, with no consensus on the optimal pulse sequence (s) or signal model (s). In general, the trend has been towards data-intensive fitting of highly parameterized models. We take the opposite approach and show that a judicious sub-sample of diffusion
exchange spectroscopy (DEXSY) data can be used to robustly quantify
exchange, as well as restriction, in a data-efficient manner. This sampling produces a ratio of two points per mixing time: (i) one point with equal diffusion weighting in both encoding periods, which gives maximal exchange contrast, and (ii) one point with the same total diffusion weighting in just the first encoding period, for normalization. We call this quotient the Diffusion
EXchange Ratio (DEXR). Furthermore, we show that it can be used to probe time-dependent diffusion by estimating the velocity autocorrelation function (VACF) over intermediate to long times (∼2-500ms). We provide a comprehensive theoretical framework for the design of DEXR experiments in the case of static or constant gradients. Data from Monte Carlo simulations and experiments acquired in fixed and viable ex vivo neonatal mouse spinal cord using a permanent magnet system are presented to test and validate this approach. In viable spinal cord, we report the following apparent parameters from just 6 data points: τk=17±4ms, fNG=0.72±0.01, Reff=1.05±0.01μm, and κeff=0.19±0.04μm/ms, which correspond to the exchange time, restricted or non-Gaussian signal fraction, an effective spherical radius, and permeability, respectively. For the VACF, we report a long-time, power-law scaling with ≈t-2.4, which is approximately consistent with disordered domains in 3-D. Overall, the DEXR method is shown to be highly efficient, capable of providing valuable quantitative diffusion metrics using minimal MR data.